A suitable contacting scheme for p-(Al)GaN facilitating quick feedback and accurate measurements is proposed in this study. 22 nm p⁺-GaN followed by 2 nm p-In_0.2Ga_0.8N was grown on p-type layers by metal-organic chemical vapor deposition. Samples were then cut into squares after annealing and contact electrodes using In balls were put at the corners of the squares. Good linearity between all the electrodes was confirmed inI–V curves during Hall measurements even with In metal. Serval samples taken from the same wafer showed small standard deviation of ~ 4% for resistivity, Hall mobility and hole concentration. The influence of contact layer on the electrical characteristics of bulk p-type layers was then investigated by step etching technique using inductively coupled plasma etching and subsequent Hall-effect measurements. Identical values could be obtained consistently when a 28 nm non-conductive layer thickness at the surface was taken into account. Therefore, the procedures for evaluating the electrical properties of GaN-based p-type layers just using In balls proposed in this study are shown to be quick and useful as for the other conventional III–V materials.

Specific contact resistance ρc to p-GaN was measured for various structures of Ni/Pd-based metals and thin (20–30 nm thick) p-InGaN/p⁺-GaN contacting layers. The effects of surface chemical treatment and annealing temperature were examined. The optimal annealing temperature was determined to be 550 °C, above which the sheet resistance of the samples degraded considerably, suggesting that undesirable alloying had occurred. Pd-containing metal showed ~35% lower ρc compared to that of single Ni. Very thin (2–3.5 nm thick) p-InGaN contacting layers grown on 20–25 nm thick p⁺-GaN layers exhibited one to two orders of magnitude smaller values of ρc compared to that of p⁺-GaN without p-InGaN. The current density dependence of ρc, which is indicative of nonlinearity in current-voltage relation, was also examined. The lowest ρc achieved through this study was 4.9 × 10^–5 Ω·cm² @J = 3.4 kA/cm².Specific contact resistance ρc to p-GaN was measured for various structures of Ni/Pd-based metals and thin (20–30 nm thick) p-InGaN/p⁺-GaN contacting layers. The effects of surface chemical treatment and annealing temperature were examined. The optimal annealing temperature was determined to be 550 °C, above which the sheet resistance of the samples degraded considerably, suggesting that undesirable alloying had occurred. Pd-containing metal showed ~35% lower ρc compared to that of single Ni. Very thin (2–3.5 nm thick) p-InGaN contacting layers grown on 20–25 nm thick p⁺-GaN layers exhibited one to two orders of magnitude smaller values of ρc compared to that of p⁺-GaN without p-InGaN. The current density dependence of ρc, which is indicative of nonlinearity in current-voltage relation, was also examined. The lowest ρc achieved through this study was 4.9 × 10^–5 Ω·cm² @J = 3.4 kA/cm².

Absorption induced by activated magnesium (Mg) in a p-type layer contributes considerable optical internal loss in GaN-based laser diodes (LDs). An LD structure with a distributed polarization doping (DPD) p-cladding layer (CL) without intentional Mg doping was designed and fabricated. The influence of the anti-waveguide structure on optical confinement was studied by optical simulation. The threshold current density, slope efficiency of LDs with DPD p-CL, and Mg-doped CL, respectively, were compared. It was found that LDs with DPD p-CL showed lower threshold current density but reduced slope efficiency, which were caused by decreasing internal loss and hole injection, respectively.

Green laser diodes (LDs) still perform worst among the visible and near-infrared spectrum range, which is called the “green gap.” Poor performance of green LDs is mainly related to the p-type AlGaN cladding layer, which on one hand imposes large thermal budget on InGaN quantum wells (QWs) during epitaxial growth, and on the other hand has poor electrical property especially when low growth temperature has to be used. We demonstrate in this work that a hybrid LD structure with an indium tin oxide (ITO) p-cladding layer can achieve threshold current density as low as 1.6kA/cm2, which is only one third of that of the conventional LD structure. The improvement is attributed to two benefits that are enabled by the ITO cladding layer. One is the reduced thermal budget imposed on QWs by reducing p-AlGaN layer thickness, and the other is the increasing hole concentration since a low Al content p-AlGaN cladding layer can be used in hybrid LD structures. Moreover, the slope efficiency is increased by 25% and the operation voltage is reduced by 0.6 V for hybrid green LDs. As a result, a 400 mW high-power green LD has been obtained. These results indicate that a hybrid LD structure can pave the way toward high-performance green LDs.